This invention relates to a solid alkene polymerization catalyst components having a reduced decay-rate.
There are numerous prior art proposals to provide alkene polymerization catalysts by combining a solid component comprising at least magnesium, titanium and halogen with an activating organoaluminium compound. These may be referred to as supported coordination catalysts or catalyst systems. The activity and stereospecific performance of such compositions is generally improved by incorporating an electron donor in the solid component and by employing as a third catalyst component an electron donor which may be complexed in whole or in part with the activating organoaluminium compound.
For convenience of reference, the solid titanium-containing constituent of such catalyst systems is referred to herein as "procatalyst", the organoaluminium compound, whether used separately or partially or totally complexed with an electron donor, as "cocatalyst", and the electron donor compound, whether used separately or partially or totally complexed with the organoaluminium compound, as "selectivity control agent" (SCA).
Supported coordination catalysts of this type are generally able to produce alkene polymers in high yield and, in the case of catalysts for polymerization of propylene or higher alkenes, with high selectivity to stereoregular polymer. However, they tend to lose their advanced polymer productivity rate after a fairly short period of time. This negative phenomenon is known as "decay", c.f. L. Luciani, Angew. Makromol. Chemie, 94 (1981), p. 63-89, FIGS. 14 and 15.
Proposals to reduce the undesirably high decay rate ar known from the prior art. EP-B 18025 relates to a slowly decaying catalyst system in which the procatalyst comprises a magnesium halide support, an electron donor and a phenoxy titanium halide. Another catalyst system having a reduced decay rate comprises a procatalyst based upon magnesium halide, titanium halide and di-isobutyl phthalate and a co-catalyst comprising trialkyl aluminium which is complexed with phenyltriethoxysilane (EP-B 45977). In the latter catalyst system the reduced decay rate is accompanied by a significant narrowing of the molecular weight distribution of the polymer. In the catalyst systems disclosed in EP-B 18025 the advantage of reduced decay rate is obtained at the detriment of a severe loss of initial polymerization rate. FIG. 1 of EP-B 18025 shows a loss of at least 50%. As a result thereof, the amount of polymer produced with a conventional prior art catalyst remains larger than that obtained with the low decay rate catalyst during a polymerization period of two hours or more under the polymerization conditions relevant to FIG. 1 of EP-B 18025.
The present invention seeks to find a solution to the aforesaid problems. It is an object of this invention to conceive catalyst systems that have in combination: a) a reduced decay rate, b) a less severe reduction of initial polymerization rate, and c) the ability to produce polymers with a wider molecular weight distribution than is obtainable with catalyst systems comprising di-esters of dihydric aromatic carboxylic acids and silane selectivity control agents.